Function computeVertexNormal

packages/melonjs/src/math/vertex.ts:271–315

(
	points: XYPoint[],
	index: number,
	out: XYPoint = _defaultNormal,
)

 * @returns unit normal
 */
export function computeVertexNormal(
	points: XYPoint[],
	index: number,
	out: XYPoint = _defaultNormal,
): XYPoint {
	let nx = 0;
	let ny = 0;
	const last = points.length - 1;
	const p = points[index];

	if (index < last) {
		const dx = points[index + 1].x - p.x;
		const dy = points[index + 1].y - p.y;
		const len = Math.sqrt(dx * dx + dy * dy);
		if (len > 0) {
			const invLen = 1 / len;
			nx -= dy * invLen;
			ny += dx * invLen;
		}
	}
	if (index > 0) {
		const dx = p.x - points[index - 1].x;
		const dy = p.y - points[index - 1].y;
		const len = Math.sqrt(dx * dx + dy * dy);
		if (len > 0) {
			const invLen = 1 / len;
			nx -= dy * invLen;
			ny += dx * invLen;
		}
	}

	const nlen = Math.sqrt(nx * nx + ny * ny);
	if (nlen > 0) {
		const invNlen = 1 / nlen;
		nx *= invNlen;
		ny *= invNlen;
	} else {
		nx = 0;
		ny = 1;
	}

	out.x = nx;
	out.y = ny;
	return out;
}

/**
 * 2D cross product of vectors (p1-p0) and (p2-p0).